In order to satisfy the reliability and operating lifetime requirements of very long distance transmission links, it has become necessary to provide optical fiber transmission systems with a degree of redundancy, in particular concerning the laser diodes which are fitted in the repeaters. This redundancy is obtained by initially equipping each repeater with several spare laser diodes which are subsequently brought into operation one by one by a switching circuit. Such a configuration requires means for remotely controlling the repeater switching circuits from the line's terminal equipment, together with remote signalling means from each repeater for informing the terminal equipment on the state of the laser diodes in service.
U.S. Pat. No. 4,281,416 describes an optical fiber digital link fitted with remote control enabling the laser diode in service in each repeater to be replaced by a spare laser diode, together with remote signalling specific to each repeater enabling an alarm to be transmitted when the bias current of the laser diode in service exceeds a threshold. The remote control signal is constituted by a digital message which is of the same nature as the digital signal transmitted over the link, and which is inserted into the transmitted traffic by the remote surveillance terminal. This digital message is repeated consecutively several times in order to avoid spurious operation, and it is constituted by a portion which identifies the repeater concerned, and a portion which identifies which laser diode is to be put into service in that repeater. The remote signalling signal is a pattern constituted by a pseudo-random binary string whose initial value identifies the source repeater.
This remote signalling has the drawback of requiring each repeater to include a source of pseudo-random strings capable of operating at the digital signal bit rate which, in an optical fiber digital transmission system, is an extremely high rate which must be provided using ECL technology, and which therefore consumes a great deal of energy. This constitutes a significant increase in the total energy consumed by the repeater, and above all it constitutes a considerable increase in the heat which the repeater must be capable of dissipating.
The above-described prior art system also suffers from the drawback of interrupting the link for a period of time which is long enough to cause synchronization to be lost in the digital data stream hierarchy, which means that such signalling can only be used for transmitting major alarms.
Preferred implementations of the present invention avoid these drawbacks and provide remote signalling by momentarily substituting a remote signalling message for the data normally conveyed by the link using a minimum of very high speed circuits for generating the messages and without disturbing the digital stream and its processing except during the time a message is actually being sent.